At present, n-type thin film transistor (TFT) has been well developed. However, in OLED, a threshold voltage fluctuation of the n-type TFT decreases a stability of light brightness. P-type TFT can be used to effectively avoid this problem and improve the luminous efficacy of the OLED.
Stannous oxide (i.e., tin(II) oxide, SnO) is an important p-type semiconductor whose electronic structure is different from that of most of other oxide semiconductors The valence band of SnO is formed from a hybridization of O's 2p orbit and Sn's 5s orbit. As a result, a hole mobility of SnO is higher than most of the other oxides, which can improve the mobility of the p-type TFT.
The sputtering target made of Sn metal or SnO can be used to sputter the SnO thin film. However, when the sputtering target is Sn metal, sputtering power and substrate temperature are limited due to the low melting point of the Sn metal. A high sputtering power may melt the Sn metal into liquid form. However, a low sputtering power reduces the sputtering efficiency. In addition, sputtering the Sn metal target easily results a SnO2 film, not the SnO film, so the sputtering conditions are strictly confined. On the other hand, SnO has a relative high resistance. Therefore, when using the SnO as the sputtering target, the SnO thin film can be sputtered only by a radio frequency magnetron sputtering (RF magnetron sputtering) method, which has a low sputtering rate. In addition, the SnO target can be made by sintering SnO powders. The SnO target sintered from the SnO powders at a low temperature has a low relative density. The SnO target sintered from the SnO powders at a high temperature has a poor thermal stability. Thus, the sintering reaction is complicated, and so that components of the SnO target and the SnO thin film are difficult to precisely control.